My Hero Academia is set in a near-future world where superpowered Quirks are the norm, but exactly how those Quirks are used is up to the individual. Many Quirks are too minor to be useful for hero work or villainy, but some people are born with formidable talents and abilities. Whether powerful individuals become heroes or villains depends on their life experiences and decisions. The following villains had the Quirks to be fine heroes but chose to go down a darker path.
Himiko Toga: The Stealthy Hero
Not all heroes need to throw a punch like All Might or blow something up like Katsuki Bakugo. Nonviolent Quirks are also vital for keeping the peace and tracking criminals. Himiko Toga's abilities would be perfect for stealthy hero work. By assuming all kinds of disguises, she could "join" crime rings or crime families and gather all the intel the heroes and police need to launch effective raids or sting operations. She could also talk to the villains and ask just the right questions to draw even more intel out of them.
Twice, The Rescue Army
Twice the villain may act goofy, but he is suffering greatly from an identity crisis and other mental health issues. If he had found a healthier way to handle these issues, he could have used his body multiplication Quirk to great effect as a hero. He could corner villains and block their exit with sheer numbers. When it comes to rescue operations, Twice could make a platoon of himself and evacuate many civilians at once. If Hawks isn't around to evacuate imperiled civilians with his floating feathers, then Twice could handle the job.
Toya Setsuno, The Disarming Hero
In the original anime, Toya Setsuno is one of the Eight Bullets, a team of elite villains that serves directly under Overhaul himself. Toya can use his Larceny Quirk to telekinetically grab an object from afar and bring it into his hands, but if he became a hero, he could use his ability to disarm villains. He could yank shotguns, pistols, bomb detonators and more right out of villains' hands, as well as retrieving stolen items from afar. If a quick-footed villain is about to escape with stolen goods, such as a purse or jewelry, Toya could swipe it right from the villain's hands, and let another hero capture the villain personally.
Magne, The Attraction Hero
Magne's Quirk charges men and women with opposing poles and draws them together with magnets, even from a great distance. As a hero, Magne could use this Quirk to catch fleeing villains or rescue civilians from disasters. While using his magnetism for these purposes would take some practice, a hero Magne could pull it off. It also doesn't hurt that Magne has some impressive martial arts skills in case he needs to fight any villains. After all, Magne was able to fight Tiger of the Wild, Wild Pussycats on even terms.
Stain, The Bloodcurdling Hero
Arguably, Stain the hero killer is more of a brutal rogue or vigilante than a traditional villain. He doesn't want to rob anyone or conquer the world, and he really does respect genuine heroes, such as All Might and even Izuku Midoriya. He feels disgusted by greedy, self-centered heroes and feels compelled to weed them out. But what if he used his Bloodcurdle Quirk and techniques for good?
Stain could be highly effective at ambushing and paralyzing powerful villains all on his own, using stealth, extreme speed, favorable terrain and the element of surprise to get the most of his Quirk. If he can obtain a blood sample, then it's all over for his enemy, and this could make powerhouse villains easy to capture. If a hero really does go rogue or abuse their power, then Stain could capture (rather than kill) them with his abilities. Stain doesn't have to execute his victims; in the anime, he chose to, out of principle, but as a hero, he would show more discretion.